Conventionally, various materials have been proposed and used for photosensitive layers of electrophotographic photoreceptors. One of them is an inorganic material such as selenium, zinc oxide or cadmium sulfide, and another is an organic material. As so-called organic photoreceptors using organic materials, ones in which materials excellent in charge generating ability and materials excellent in charge transporting ability are used in combination, namely function separation type photoreceptors, occupy the main current rather than ones having a single layer structure. The function separation type photoreceptors have the advantage that the selection range of materials used for the photosensitive layers is expanded, thereby being excellent in electrophotographic characteristics such as charging property and sensitivity. The charge generating materials used for charge generating layers include bisazo pigments, phthalocyanine pigments, pyrylium pigments, perylene pigments, polycyclic quinone pigments, quinacridone pigments and indigo pigments. Further, the charge transporting materials used for charge transporting layers include pyrazoline, hydrazones and polyvinylcarbazole.
The charge generating layers are sometimes formed by using the charge generating materials alone, but binder resins are generally used in combination therewith. As the binder resins used for the charge generating layers, materials are generally selected for use which have properties as coatings such as dispersibility to the charge generating materials, stability of dispersions, adhesion to electrically conductive layers or undercoating layers, and dissolution resistance and penetration resistance to organic solvents contained in the charge transporting layers, as well as electrophotographic characteristics such as sensitivity, charging property and repetitive characteristics. Examples of the binder resins conventionally used include polycarbonates, polystyrene, polyesters, polyvinyl butyral, vinyl acetate polymers or copolymers, polyurethanes and epoxy resins.
However, the binder resins conventionally used have the disadvantage that the charge blocking is liable to take place due to their high water absorption when they are repeatedly used under the condition of high temperature and humidity for a long time, which causes a reduction in charging property and an increase in residual potential. Accordingly, the appearance of a binder which does not produce such a disadvantage has been desired.